1. Field of the Invention
The present invention relates to a printing apparatus and a printing method for correcting density variations resulting from differences in print characteristics among predetermined nozzle groups of a plurality of ink ejection nozzles.
2. Description of the Related Art
There is known an ink jet printing apparatus which includes a print head provided with a plurality of nozzles for ink ejection and ejecting ink drops to form ink dots on a print medium to print characters and images.
Nozzles differing in diameter for each position in a single substrate of a print head eject different volumes of ink according to their diameters even if other printing conditions are the same, and as a result, variations may occur in size of ink dots formed on a print medium. In addition, in the case of a print head employing a piezoelectric element which ejects ink by an applied pressure as a printing element, differences in material and working precision of the piezoelectric element may affect a displacement of the ink volume that the print head can eject. Accordingly, in a printing apparatus provided with a print head having many nozzles arranged therein, ejected ink volumes vary depending on the print characteristic of each nozzle, causing variations in size of the formed ink dots, which may result in density variations in images.
To correct such density variations, that is, differences in the ink volume used for printing, control for compensating differences in the ink volume based on the number of ink dots used for printing is conventionally known. U.S. Pat. No. 7,249,815 discloses a printing apparatus comprising a plurality of nozzles arranged according to a predetermined distribution, the plurality of nozzles having a target average droplet volume and an actual average droplet volume wherein a subset of the plurality of nozzles is sized larger than others of the plurality of nozzles, and a controller configured to selectively drive nozzles. The controller corrects print density by selecting nozzles to drive such that the actual average droplet volume is equal to the target average droplet volume.
According to the printing apparatus disclosed in U.S. Pat. No. 7,249,815, print density is corrected. On the other hand, however, a pattern formed by printed dots (hereinafter referred to as “a dot pattern”) is different from a dot pattern formed when the correction is not performed. This is because positions of dots printed on a print medium differ between the nozzles selectively driven for print density correction and the nozzles driven when the print density correction is not performed.
For this reason, the conventional technique had a problem that making a significant correction above a certain level results successfully in print density correction but disadvantageously in visual recognition of a difference in a dot pattern, leading to degradation in image quality. On the other hand, to ensure that image quality is maintained, a range of print density correction is limited.